"I made clear earlier this month that I would report back to parliament on UK force structures in Afghanistan once I had met my Nato colleagues in Seville.

Following a cabinet discussion yesterday, the Ministry of Defence is now working on the final details of changes to our force levels. When this work is complete, I will announce our decisions in the proper way, to parliament on Monday.

It had not been my intention to say anything in advance of this. However, speculation in the media over the course of today is likely to be causing concern among our forces and their families, and I feel obliged to clarify the situation.

At Seville, Nato invited all member nations to review their force levels in Afghanistan, focusing on the southern and eastern regions. The UK has always agreed that Nato needs more forces in these regions, where the Taliban challenge is most serious - in particular, for a greater manoeuvre capability.

Nato must respond to this request, or we will put at risk everything we have achieved across Afghanistan in the last five years: the stability which has brought five million refugees home, the advances in democracy, the economy, human rights and women's rights.

We have been trying hard to get other nations to live up to the joint commitment Nato made to Afghanistan and provide more forces, forces which are authorised to fight. We will continue to press. But we must be realistic.

We have decided that it is right for the UK to provide some additional forces for the southern region.

We are acutely aware that our armed forces, particularly the army, continue to operate at a high operational tempo, but we believe this additional commitment is manageable. This, like the other aspects of our decisions, is based on clear military advice."